Electrodeless high intensity discharge (HID) lamps have been described extensively in the prior art. In general, electrodeless HID lamps include an electrodeless lamp capsule containing a volatilizable fill material and a starting gas. The lamp capsule is mounted in a fixture which is designed for coupling high frequency power to the lamp capsule. The high frequency power produces a light-emitting plasma discharge within the lamp capsule. Recent advances in the application of microwave power to lamp capsules operating in the tens of watts range are disclosed in U.S. Pat. No. 5,070,277, issued Dec. 3, 1991, to Lapatovich; U.S. Pat. No. 5,113,121, issued May 12, 1992, to Lapatovich, et al.; U.S. Pat. No. 5,130,612, issued Jul. 14, 1992, to Lapatovich et al.; U.S. Pat. No. 5,144,206, issued Sep. 1, 1992, to Butler et al.; and U.S. Pat. No. 5,241,246, issued Aug. 31, 1993, to Lapatovich, et al. As a result, compact electrodeless HID lamps and associated applicators have become practical.
The above patents disclose small, cylindrical lamp capsules wherein high frequency energy is coupled to opposite ends of the lamp capsule with a 180.degree. phase shift. The applied electric field is generally colinear with the axis of the lamp capsule and produces a substantially linear discharge within the lamp capsule. The fixture for coupling high frequency energy to the lamp capsule typically includes a planar transmission line, such as a microstrip transmission line, with electric field applicators, such as helices, cups or loops, positioned at opposite ends of the lamp capsule. The microstrip transmission line couples high frequency power to the electric field applicators with a 180.degree. phase shift. The lamp capsule is typically positioned in a gap in the substrate of the microstrip transmission line and is spaced above the plane of the substrate by a few millimeters, so that the axis of the lamp capsule is colinear with the axes of the field applicators.
Electrodeless high intensity discharge lamps have found potential applications, for example, in automotive lighting, surgical lighting, fiber optics illumination and as light sources for matrix projection displays. Their unique properties, derived from the lack of electrodes, make them particularly suitable for applications requiring long life, good lumen maintenance and high color stability. To be commercially viable in many of these applications, electrodeless HID lamps require a method of relighting the lamp upon a momentary loss of power. The loss of power may be due to the operator attempting to quickly relight the lamp or due to a momentary uncontrolled break in the supply of power to the lamp. Consumers expect tungsten light sources to quickly reestablish light output upon reapplication of power, and expect electrodeless HID lamps to do the same.
High intensity discharge lamps suffer from a relighting difficulty after power interruption due to the extremely high vapor pressure of the constituent gases resulting from high lamp temperature. During operation, HID lamp arc tubes operate at temperatures that exceed 750.degree. C. Metal halide lamps, for example, containing mercury exhibit internal pressures that range from 1 to 200 atmospheres. Room temperature pressures inside the arc tube are typically 10.sup.-5 atmospheres of mercury vapor and 1 to 500 Torr of starting gas, typically argon or xenon. The difference is due to the high increase in mercury vapor pressure with arc tube temperature. As pressure inside the arc tube increases, it becomes more and more difficult to start a discharge. This is a particular problem when power is momentarily interrupted. The lamp will not restart until it cools down close to room temperature. Cooling may take up to several minutes, during which no light is emitted from the lamp.
Hot relight has long been an issue with electroded metal halide lamps. In this case, application of extremely high voltage pulses (40 kV) after lamp turnoff produces conditions suitable for a rapid hot relight. Voltage pulses of this magnitude are required to break down the high pressure gas and establish conditions which allow current to flow between electrodes. High voltage is not the only requirement for restarting hot lamps at high pressure. The initial high voltage pulses produce gas breakdown and corona formation from the electrodes. This is followed in time by streamer formation eventually transversing the gap between the electrodes. These are high impedance phases of hot relight. If the power delivered to the electrodes is sufficient and the voltage is high enough, a spark channel is eventually formed, which allows sufficient current flow to eventually reestablish the arc. The spark channel phase is also a relatively high voltage phase but is lower in impedance, therefore requiring higher current from the voltage source. The most common way of producing conditions that ensure hot restart is to generate a long series of high voltage pulses that overvoltage the arc tube and force it to transition through each starting phase until the conventional ballast supply can sustain an arc. A final arc stage is established when sufficient energy is transferred to the electrodes during the spark channel phase.
The generation of sequential high voltage pulses is not trivial. Such an igniter is costly and creates a safety problem. The 40 kV pulses can ionize air, causing electrical failures, destroying CMOS circuitry and presenting a safety hazard for people in contact with the lamp.
Electrodeless HID lamps have no electrodes and therefore provide no way of injecting electrons directly into the arc tube. Hot relight of electrodeless HID lamps can be achieved with high voltage pulses, but these pulses are capacitively coupled to the discharge. The capacitively coupled pulse field and the applied microwave field are required for breakdown and hot relight. Generally, this is an unacceptable approach, since it is difficult to shield the microwave output transistors in the lamp ballast from the high voltage pulse fields, often resulting in the destruction of the output transistors.
U.S. Pat. No. 5,287,039 issued Feb. 15, 1994 to Gregor, et al discloses a slow cool down method of restarting electrodeless HID lamps. The disclosed method allows instant relight at any time after turnoff. This method requires the power system to remain on during the cooling time and covers the situation where the lamp is intentionally turned off and then quickly turned back on. It does not cover the case where total power to the lighting system is momentarily interrupted or removed. Continuous cooling of relatively high power electrodeless discharge lamps during operation is disclosed in U.S. Pat. No. 5,334,913 issued Aug. 2, 1994 to Ury, et al; U.S. Pat. No. 5,404,076 issued Apr. 4, 1995 to Dolan et al and by B. P. Turner, et al in "Sulphur Lamps-Progress In Their Development" Ref. No. 87, 1995 IES Annual Conference, pages 660-672.